A method for preparing near-stoichiometric silicon carbide fibers

Abstract

A method for preparing near-stoichiometric silicon carbide fibers includes the following steps: (1) synthesizing polycarbosilane from polydimethylsilane using high-pressure or atmospheric-pressure pyrolysis; (2) spinning the polycarbosilane into fibers using melt spinning technology; (3) cross-linking the polycarbosilane-spun fibers using electron beam-assisted active atmosphere cross-linking technology to obtain polycarbosilane-spun fibers; and (4) subjecting the cross-linked polycarbosilane fibers to inorganic conversion in a hydrogen atmosphere to obtain near-stoichiometric silicon carbide fibers. This invention uses electron beam-assisted active atmosphere cross-linking technology to cross-link PCS fibers and then performs inorganic conversion in a hydrogen atmosphere to obtain near-stoichiometric silicon carbide fibers. This not only improves the cross-linking efficiency but also effectively reduces the preparation cost of near-stoichiometric silicon carbide fibers. The method is simple, environmentally friendly, and suitable for industrial-scale production.

Description

Technical Field

[0001] This invention relates to a method for preparing near-stoichiometric silicon carbide fibers, and more particularly to a method for preparing near-stoichiometric silicon carbide fibers using electron beam-assisted active atmosphere crosslinking technology. Background Technology

[0002] Silicon carbide fibers possess excellent properties such as high temperature resistance and oxidation resistance, and have broad application prospects in fields such as aviation, aerospace, engines, nuclear cladding materials and components. Globally, Japan, the United States, and China have been conducting research on the preparation and application of silicon carbide fibers for many years, and have already achieved industrialization.

[0003] There are various types of silicon carbide fibers available, and different silicon carbide fibers have different oxygen and carbon contents in their chemical composition, resulting in different properties. Among them, near-stoichiometric silicon carbide fibers have the best properties in terms of high temperature resistance, oxidation resistance, and corrosion resistance.

[0004] There are various existing methods for preparing silicon carbide fibers, which can be broadly classified into three categories: chemical vapor deposition, activated carbon fiber conversion, and precursor conversion.

[0005] (1) Chemical vapor deposition method. Its basic principle is to deposit silicon carbide on a continuous tungsten wire or carbon wire core material using chemical vapor deposition technology. The fibers prepared by this method have high purity and good properties such as high temperature strength, creep resistance and stability. However, the fiber diameter is relatively large and difficult to weave. In addition, the equipment cost is high and the production efficiency is low, so industrialization is greatly limited.

[0006] (2) Activated carbon fiber conversion method: The basic principle is to react activated carbon fiber with silicon dioxide gas at high temperature to prepare silicon carbide fiber. The silicon carbide fiber obtained by this method has a low cost, but the presence of micropores and cracks in the fiber greatly affects the fiber performance.

[0007] (3) Precursor conversion method: The basic principle of this method is to prepare silicon carbide fibers through processes such as precursor filamentation, infusibility of the precursor filament, and inorganication of the infusible fiber, based on precursor synthesis. It is currently the most ideal method for preparing silicon carbide fibers. All industrially produced silicon carbide fibers in the world are produced using the precursor conversion method.

[0008] The precursor conversion method for preparing silicon carbide fibers involves several different processes. The main difference lies in the crosslinking technology. One method uses oxidative crosslinking techniques (such as air-insoluble crosslinking) to produce first-generation silicon carbide fibers with an oxygen content of approximately 10-30 wt% (e.g., Nicalon and KD-I silicon carbide fibers). However, the excessively high oxygen content in these fibers results in insufficient high-temperature resistance and oxidation resistance. A second method uses electron beam crosslinking technology to produce carbon-rich silicon carbide fibers with an oxygen content below 1 wt% (e.g., Hi-Nicalon and Syllamic silicon carbide fibers). A third method uses non-oxygen-active atmosphere crosslinking technology, which can also produce carbon-rich silicon carbide fibers with an oxygen content below 1 wt%. These low-oxygen-content silicon carbide fibers exhibit significantly improved temperature resistance and oxidation resistance, and can be termed second-generation silicon carbide fibers. However, because the fiber is still rich in carbon, its temperature resistance and oxidation resistance are limited (see H. Ichikawa, M. Takeda, J. Sakamoto, A. Saeki, Process for Producing Silicon Carbide Fibers, US Patent, US5824281, 1998, 10. and M. Sugimoto, K. Okamura, T. Seguchi, Reaction Mechanisms of Silicon Carbide Fiber Synthesis by Heat Treatment of Polycarbosilane Fibers Cured by Radiation: I. Evolved Gas Analysis, J. Am. Ceram. Soc., 1995, 78(4), 1013-1017; and CN109485388A).

[0009] The above methods cannot produce silicon carbide fibers with near stoichiometric ratios.

[0010] The current method for preparing near-stoichiometric silicon carbide fibers is to use hydrogenation decarburization technology. That is, hydrogen gas is introduced during the inorganication process of cross-linked fibers, so that carbon-containing free radicals such as methyl groups generated during the inorganication process combine with hydrogen atoms and are then eliminated in the form of gases such as methane. By controlling the degree of reaction, near-stoichiometric silicon carbide fibers can be prepared (see M. Takeda, A. Saeki, J. Sakamoto, Y. Imai, H. Ichikawa, Effect of HydrogenAtmosphere on Pyrolysis of Cured Polycarbosilane Fibers, J. Am. Ceram. Soc., 2000, 83(5), 1063-1069).

[0011] The basic principle of electron beam crosslinking technology is to generate free radicals by continuously irradiating the precursor fiber with an electron beam, and then achieve crosslinking of the precursor fiber by combining the free radicals with adjacent free radicals. Due to the large dose (12~35 MGy) and long time (greater than 10 hours) of irradiation, the preparation cost is relatively high.

[0012] The basic principle of non-oxygen reactive atmosphere crosslinking technology is that crosslinking is achieved by chemically reacting the active groups in the precursor fiber with the reactive atmosphere. Therefore, the uniformity of the reaction and the reaction process are difficult to control, which is not environmentally friendly and the preparation efficiency is low (see CN109485388A). Summary of the Invention

[0013] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a near-stoichiometric method for preparing silicon carbide fibers with high preparation efficiency, low preparation cost, simple process, environmental friendliness, and suitability for industrial production.

[0014] The technical solution adopted by this invention to solve its technical problem is as follows: A method for preparing near-stoichiometric silicon carbide fibers, comprising the following steps:

[0015] (1) Polycarbosilane is synthesized by using polydimethylsilane (PDMS) as raw material and preparing polycarbosilane (PCS) by high-pressure cracking or atmospheric-pressure cracking;

[0016] (2) Polycarbosilane is spun into fibers (PCS fibers) by melt spinning technology.

[0017] (3) Polycarbosilane was spun into fibers and cross-linked using electron beam-assisted active atmosphere cross-linking technology to obtain polycarbosilane cross-linked fibers (PCS cross-linked fibers).

[0018] (4) The polycarbosilane cross-linked fiber was inorganically converted in a hydrogen atmosphere to obtain near stoichiometric silicon carbide fiber.

[0019] Furthermore, in step (1), the polycarbosilane is a polymer with a main chain structure of -[(R1)Si(R2)-CH2]- and is solid at room temperature, wherein R1 and R2 are alkyl, phenyl or H with ≤5 carbon atoms, and R1 and R2 can be the same or different.

[0020] Furthermore, in step (1), the high-pressure pyrolysis temperature is 300~800℃, the pressure is 0.12~10Mpa, and the time is 1~100hr; the atmospheric pressure pyrolysis temperature is 300~750℃, and the time is 5~100hr.

[0021] Furthermore, in step (1), the temperature of the high-pressure pyrolysis is preferably 350~700℃, the pressure is preferably 0.15~72Mpa, and the time is preferably 5~80hr.

[0022] Furthermore, in step (1), the temperature of the atmospheric pressure pyrolysis is preferably 350-650℃, and the time is preferably 10-80hr.

[0023] Furthermore, in step (2), the temperature of the melt spinning is 150~400℃.

[0024] Furthermore, in step (2), the temperature of the melt spinning is preferably 180~380℃.

[0025] Furthermore, in step (3), the active atmosphere refers to a gas that is gaseous at room temperature and normal pressure and can form ≥2 free radicals without oxygen elements under electron beam irradiation conditions.

[0026] Furthermore, in step (3), the active atmosphere includes, but is not limited to, one of ethylene, propylene, acetylene, propyne, butadiene, boron trichloride, etc., or a mixture of two or more of these gases.

[0027] Furthermore, in step (3), the dose of electron beam irradiation is 1kGy~10Gy (preferably 1.5kGy-5MGy; more preferably 2.0kGy-3.0MGy), and the irradiation time is 15min~5hr, preferably 30min~3hr.

[0028] Furthermore, in step (4), the hydrogen atmosphere refers to a mixed atmosphere of hydrogen with nitrogen, argon or helium, and the volume ratio of hydrogen to the mixed gas is 5 to 100% (preferably 15 to 80%).

[0029] Furthermore, in step (4), the inorganicization refers to the process of converting the polycarbosilane crosslinked fiber from organic to inorganic and the formation and growth of grains. The inorganicization temperature is 900~1900℃, the holding time is 1min~30hr, and the heating time from room temperature to inorganicization temperature is 10min~50hr.

[0030] Furthermore, in step (4), the inorganication temperature is preferably 1250~1800℃, the heat preservation time is preferably 2min~25hr, and the heating time from room temperature to inorganication temperature is preferably 30min~40hr.

[0031] The principle of this invention is as follows: Based on the synthesis of PCS precursor, PCS is spun into fibers. Through electron beam irradiation, the precursor polysilicon carbide fibers and active gases simultaneously form free radicals. The precursor polysilicon carbide fibers are then cross-linked through the combination of free radicals. Finally, the cross-linked polysilicon carbide fibers are inorganically converted in a mixed gas atmosphere of hydrogen and inert atmosphere to prepare near stoichiometric silicon carbide fibers.

[0032] The beneficial effects of this invention are as follows: Electron beam-assisted active atmosphere crosslinking technology can effectively achieve uniform crosslinking of precursor polysilicon carbide fibers. Decarburization under a hydrogen atmosphere removes excess carbon from the fibers, thereby achieving a near-stoichiometric ratio of carbon and silicon atoms. The inventors have combined electron beam irradiation with an oxygen-free active atmosphere to develop an electron beam-assisted active atmosphere crosslinking technology for preparing crosslinked polysilicon carbide fibers. Specifically, electron beam irradiation causes the precursor fibers and the active gas to simultaneously form free radicals, which then combine to achieve crosslinking of the precursor fibers. The PCS crosslinked fibers prepared using this invention achieve inorganication of the crosslinked fibers while decarburizing under a hydrogen atmosphere, thus producing near-stoichiometric silicon carbide fibers. This invention offers high production efficiency (more than 20% higher), reduces the production cost of near-stoichiometric silicon carbide fibers (more than 30% lower), and features a simple process that is environmentally friendly and suitable for industrial production. Attached Figure Description

[0033] Figure 1 The image shows the morphology of the silicon carbide fibers prepared in Example 1.

[0034] Figure 2 The image shows the morphology of the silicon carbide fibers prepared in Example 2.

[0035] Figure 3 The image shows the morphology of the silicon carbide fibers prepared in Example 3. Detailed Implementation

[0036] The present invention will be further described below with reference to the embodiments.

[0037] The raw materials used in the embodiments of the present invention are all obtained through conventional commercial channels.

[0038] The atmospheric pressure pyrolysis device used in the embodiments is a glass system built by the inventors themselves; the spinning equipment used is a non-standard equipment developed by Jiangxi Donghua Machinery Co., Ltd.; the 1.2MV-100KW electron accelerator used is produced by Wuhan Aibang High Energy Technology Co., Ltd.; the inorganic furnace is model H(D)SC-050511, produced by Hunan Dingli Technology Co., Ltd.

[0039] The strength of the silicon carbide fibers obtained in each embodiment was determined according to the method specified in GB / T34520.5-2017, the modulus was determined according to the same conventional method, the density was tested according to the method specified in GB / T34520.3-2017, the C / Si ratio was tested according to the method specified in GB / T34520.9-2021, and the oxygen content was tested according to the method specified in GB / T34520.8-2021. Example 1

[0040] (1) PCS was synthesized from PDMS using atmospheric pressure pyrolysis technology: 5 kg of PDMS was placed in an atmospheric pressure pyrolysis device, and the system was heated to 550 °C at full speed under nitrogen protection and held for 50 hours. Then it was naturally cooled to room temperature to obtain PCS. The structural formula of the obtained PCS is -[(CH3)Si(H)-CH2]-, and the number average molecular weight is 1520.

[0041] (2) PCS was spun into fibers at a spinning temperature of 320℃ using melt spinning technology to obtain PCS filaments. The average diameter of the obtained filament fibers was 15μm.

[0042] (3) Place 5g of PCS precursor fiber in the irradiation chamber, evacuate and replace the ethylene gas, then turn on the electron accelerator. The irradiation dose is 3MGy and the irradiation time is 2.5hr to obtain PCS cross-linked fiber.

[0043] (4) The PCS cross-linked fiber was placed in an inorganic furnace and heated from room temperature to 1350°C for 20 hours in a hydrogen atmosphere (hydrogen accounts for 40% of the volume of the hydrogen and nitrogen mixture) and kept at that temperature for 1 hour to obtain near stoichiometric silicon carbide fiber.

[0044] The near-stoichiometric silicon carbide fiber morphology photographs prepared in this embodiment are shown below. Figure 1 As shown.

[0045] The silicon carbide fiber obtained in this embodiment has a strength of 3.5 GPa, a modulus of 350 GPa, and a density of 2.77 g / cm³. 3The C / Si ratio was 1.12 and the oxygen content was 0.41 wt%. The strength retention rate was 91% after 1 hour of treatment in a nitrogen environment at 1500℃ and 86.3% after 1 hour of treatment in air at 1200℃. Example 2

[0046] Steps (1) and (2) are the same as steps (1) and (2) in Example 1.

[0047] (3) Place 5g of the PCS precursor obtained in Example 1 in the irradiation chamber, evacuate and replace the acetylene gas, then turn on the electron accelerator. The irradiation dose is 1.5MGy and the irradiation time is 40min to obtain PCS cross-linked fiber.

[0048] (4) The PCS cross-linked fiber was placed in an inorganic furnace and heated from room temperature to 1450°C for 20 hours in a hydrogen atmosphere (the volume ratio of hydrogen to nitrogen mixture was 20%) and kept at that temperature for 1 hour to obtain near stoichiometric silicon carbide fiber.

[0049] The near-stoichiometric silicon carbide fiber morphology photographs prepared in this embodiment are shown below. Figure 2 As shown.

[0050] The resulting silicon carbide fibers had a strength of 3.8 GPa, a modulus of 280 GPa, and a density of 2.87 g / cm³. 3 The C / Si atomic ratio is 1.08 and the oxygen content is 0.37wt%. The strength retention rate after 1 hour of treatment in a nitrogen environment at 1500℃ is 89.4%, and the strength retention rate after 1 hour of treatment in air at 1200℃ is 87.2%. Example 3

[0051] (1) PCS was synthesized from PDMS using high-pressure pyrolysis technology: 2 kg of PDMS was placed in a high-pressure pyrolysis reactor, and nitrogen was replaced by vacuum at a pressure of 2 MPa. The system was then rapidly heated to 600 °C and kept at that temperature for 50 hours. Finally, the system was allowed to cool naturally to room temperature to obtain PCS with a number average molecular weight of 1660.

[0052] (2) The PCS was spun into fibers using melt spinning technology at a spinning temperature of 360℃, and the average diameter of the resulting fibers was 18μm.

[0053] (3) Place 5g of PCS precursor fiber in the irradiation chamber, evacuate and replace butadiene gas, then turn on the electron accelerator. The irradiation dose is 5MGy and the irradiation time is 2.5hr to obtain PCS cross-linked fiber.

[0054] (4) The PCS cross-linked fiber was placed in an inorganic furnace and heated from room temperature to 1600°C for 15 hours in a hydrogen atmosphere (hydrogen accounts for 15% of the volume of the hydrogen and helium mixture) and held for 0.5 hours to obtain near stoichiometric silicon carbide fiber.

[0055] The near-stoichiometric silicon carbide fiber morphology photographs prepared in this embodiment are shown below. Figure 3 As shown.

[0056] The resulting silicon carbide fibers had a strength of 3.2 GPa, a modulus of 390 GPa, and a density of 2.91 g / cm³. 3 The C / Si atomic ratio is 0.98 and the oxygen content is 0.33 wt%. The strength retention rate is 91% after 1 hour of treatment in a nitrogen environment at 1500℃ and 83.6% after 1 hour of treatment in air at 1200℃.

[0057] The processes described in each embodiment are environmentally friendly and suitable for industrial production.

Claims

1. A method for preparing near-stoichiometric silicon carbide fibers, characterized in that, Includes the following steps: (1) Polycarbosilane is synthesized by using polydimethylsilane as raw material and preparing polycarbosilane (PCS) by high pressure pyrolysis or atmospheric pressure pyrolysis; (2) Polycarbosilane is spun into fibers by melt spinning technology to obtain polycarbosilane fibers; (3) The polycarbosilane fiber was cross-linked by electron beam irradiation-assisted active atmosphere cross-linking technology to obtain polycarbosilane cross-linked fiber; The active atmosphere refers to a gaseous gas that is gaseous at room temperature and normal pressure and can form ≥2 free radicals under electron beam irradiation conditions, and is free of oxygen; specifically, it is one of ethylene, propylene, acetylene, propyne, butadiene, boron trichloride, or a mixture of two or more of these gases. The electron beam irradiation dose is 1 kGy to 10 mgy; the irradiation time is 15 min to 5 hr. (4) Polycarbosilane cross-linked fibers are inorganically converted in a hydrogen atmosphere to obtain near stoichiometric silicon carbide fibers; The hydrogen atmosphere refers to a mixed atmosphere of hydrogen with nitrogen, argon, or helium, with hydrogen accounting for 5-100% of the volume of the mixed gas; the inorganication refers to the process of converting polycarbonyl silane cross-linked fibers from organic to inorganic and the formation and growth of grains, with the inorganication temperature being 900-1900℃, the holding time being 1 min-30 hr, and the heating time from room temperature to the inorganication temperature being 10 min-50 hr.

2. The method for preparing near-stoichiometric silicon carbide fibers according to claim 1, characterized in that, In step (1), the polycarbosilane is a polymer with a main chain structure of -[(R1)Si(R2)-CH2]- and is solid at room temperature, wherein R1 and R2 are alkyl, phenyl or H with ≤5 carbon atoms, and R1 and R2 can be the same or different.

3. The method for preparing near-stoichiometric silicon carbide fibers according to claim 1, characterized in that, In step (1), the method for synthesizing polycarbosilane is to prepare polycarbosilane by high-pressure cracking or atmospheric-pressure cracking using polydimethylsilane as raw material.

4. The method for preparing near-stoichiometric silicon carbide fibers according to claim 1, characterized in that, In step (1), the high-pressure pyrolysis temperature is 300~800℃, the pressure is 0.12~10Mpa, and the time is 1~100hr.

5. The method for preparing near-stoichiometric silicon carbide fibers according to claim 4, characterized in that, In step (1), the high-pressure pyrolysis temperature is 350~700℃, the pressure is 0.15~7.2Mpa, and the time is 5~80hr.

6. The method for preparing near-stoichiometric silicon carbide fibers according to claim 1, characterized in that, In step (1), the atmospheric pressure pyrolysis temperature is 300~750℃ and the time is 5~100hr.

7. The method for preparing near-stoichiometric silicon carbide fibers according to claim 6, characterized in that, In step (1), the atmospheric pressure pyrolysis temperature is 350-650℃ and the time is 10-80hr.

8. A method for preparing near-stoichiometric silicon carbide fibers according to any one of claims 1-7, characterized in that, In step (2), the temperature of the melt spinning is 150~400℃.

9. The method for preparing near-stoichiometric silicon carbide fibers according to claim 8, characterized in that, In step (2), the temperature of the melt spinning is 180~380℃.

10. A method for preparing near-stoichiometric silicon carbide fibers according to any one of claims 1-7, characterized in that, In step (3), the dose of electron beam irradiation is 2.0 kGy to 3.0 mgy; the irradiation time is 30 min to 3 hr.

11. A method for preparing near-stoichiometric silicon carbide fibers according to any one of claims 1-7, characterized in that, In step (4), in the mixed atmosphere of hydrogen with nitrogen, argon or helium, the volume ratio of hydrogen to the mixed gas is 15-80%; the inorganication temperature is 1250-1800℃, the holding time is 2min-25hr, and the heating time from room temperature to inorganication temperature is 30min-40hr.

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